One can also force the implementation by using the "useXXX" static methods:
useConsoleLoggers(), useVerboseConsoleLoggers(), useJdkLoggers()
and useSl4jLoggers() (which may throw an Exception if the library isn't on the
classpath). Note that the system property method above is preferred, as no cleanup of
the logger factory initialized at startup is attempted by the useXXX methods.

useConsoleLoggers

public static final void useConsoleLoggers()

Force the usage of Console-based Loggers, even if SLF4J is available
on the classpath. Console loggers will output ERROR and
WARN levels to System.err and levels below to
System.out. All levels except TRACE and DEBUG are
considered enabled. TRACE and DEBUG are omitted.

The previously active logger factory is simply replaced without
any particular clean-up.

useVerboseConsoleLoggers

public static final void useVerboseConsoleLoggers()

Force the usage of Console-based Loggers, even if SLF4J is available
on the classpath. Console loggers will output ERROR and
WARN levels to System.err and levels below to
System.out. All levels (including TRACE and DEBUG) are considered enabled.

The previously active logger factory is simply replaced without
any particular clean-up.